During aging, the loss of stem cell function is thought to play a major role in organ degeneration and dysfunction. Therefore, strategies capable of restoring stem cells may lead to novel and effective therapies for a wide-range of degenerative disorders. Several cellular processes have been found to drive the aging process, but the precise mechanisms responsible for changes in stem cells are relatively unknown. The disruption of protein homeostasis (i.e., proteostasis) results in the intracellular accumulation of damaged and misfolded proteins and is a major determinant of aging. The majority of studies examining proteostasis have focused on protein folding and degradation in attempts to improve the elimination of toxic protein aggregates, but the rate of protein synthesis plays a major role i maintaining the integrity of the proteome as accelerated translation rates can lead to higher rates of incorrect amino acid incorporation as well as the misfolding of nascent polypeptides. Both caloric restriction and inhibition of the mammalian Target of Rapamycin (mTOR) can decrease protein synthesis rates and it is possible that improved proteostasis plays a major role in their capacity to extend lifespan across a range of organisms. We hypothesize that both quantitative and qualitative changes in protein synthesis are responsible for the age-related loss of stem cell function. We also hypothesize that reducing protein synthesis rates will restore proteostasis and the function of stem cells from elderly individuals. In order to address these hypotheses, we will study hematopoietic stem cells (HSCs) and progenitors since age-related changes in hematopoiesis have been well described. Furthermore, we will take advantage of our unique access to human bone marrow samples from normal donors ranging in age from 18- 75 years old that will allow us to maximize the human relevance of our studies in contrast to most studies that have investigated aging within murine hematopoiesis. Accordingly, we will: (1). determine the impact of aging on HSC and progenitor protein synthesis rates and the expression of translational machinery components and (2). determine the reversibility of age-related changes in HSPC function. For studies in Aim 1 we will quantify the rates of protein synthesis and turnover and correlate these results with in vitro and in vivo functional studies. Moreover, we will carry out polysome profiling to quantify changes in the expression of components of the translational machinery that occur with aging. In Aim 2, we will determine the impact of rapamycin and the role of differentially expressed regulators of translational on HSCs and progenitors from young and elderly donors. If successful, our findings will improve our understanding of the aging human hematopoietic system, define the role of proteostasis in aging stem cells, validate the results of murine studies regarding the effects of mTOR inhibition on HSCs, provide the basis for novel strategies to improve the function of both HSCs and other tissue-specific stem cells, and serve as the basis for the development of novel strategies to restore stem cell function in the elderly.